The present invention relates in general to the measurement of a wavefront in an optical system, and more particularly to the measurement of the wavefront in an optical system used in a photolithographic device without the need for removing the optical system, performing any analysis at a different location, and with little, if any, disruption in production associated with the photolithographic device.
Optical systems are used in many devices. In particular, optical systems are relied upon greatly in the manufacture of semiconductor devices. In one phase in the manufacturing process of semiconductor devices, photolithography is used. These photolithographic techniques are used to project the image of a reticle containing a circuit pattern thereon onto a photosensitive substrate through a projection optic. The projection optic is typically relatively complex and susceptible to changes in system performance as a result of numerous factors, such as temperature, humidity, age, and use. As more demands are placed on the projection optics in a photolithographic device, especially when the projection optic is designed to work in the ultraviolet regions of wavelength, the projection optic is susceptible to changes that may degrade performance. Due to the high degree of image quality required in photolithographic devices used to manufacture semiconductors, various measurements must be routinely taken in an effort to assure that any change in the projection optic will not effect the quality of the resulting semiconductor product. This is particularly important in view of the potentially wasted cost in time and effort of subsequent processing after the formation of an image in the photolithographic step in manufacturing of a semiconductor device. Therefore, it is essential to determine as quickly as possible when the projection optic has changed or degraded to an extent that may effect product quality. Basic performance of the projection optic is predictable from a measurement of the wavefront of the electromagnetic radiation used to project the image of a pattern onto a photosensitive substrate. Accordingly, knowledge of the wavefront is critically important. Measurement of the wavefront of the projection optic is routinely done prior to installation into a production photolithographic device or tool. These measurements are usually performed with the aid of an interferometer. However, after the projection optic is installed within the production tool or device, it is often difficult to make any further measurements of the wavefront of light or electromagnetic radiation transmitted through the projection optic. Generally, the projection optic must be removed in order to perform the required measurements, or additional measuring devices must be installed, such as an interferometer, into the photolithographic tool. Also, an image may be printed or electronically recorded and analyzed using conventional techniques in order to obtain information needed to determine system performance. However, due to the increasing need to maintain system performance with little down time and as little disruption in production as possible, there is a need to assess system performance in as near real time as possible so as to assure that performance is maintained at a high level. Therefore, there is a need to improve upon the known techniques for measuring and monitoring system performance of projection optics without the need for removal of the projection optics or installation of additional complex equipment, such as interferometers, in production photolithographic tools, or the analyzing of printed or electronically recorded images utilizing conventional techniques.
The present invention is directed to an apparatus and method for measuring and monitoring the performance of the projection optic in a photolithographic device comprising a plurality of gratings having different pitches and rotations formed or placed on a reticle in predetermined sections, in a known location, and imaged through the projection optic to be measured. Any shift in position of the grating image is detected and determined, as well as the modulation of the grating image in at least two planes with the data being used to calculate wavefront information for the projection optic.
Accordingly, it is an object of the present invention to provide wavefront information without the need for removing the projection optic from the photolithographic tool or device.
It is another object of the present invention to make metrology measurements of wavefront information quickly and in near real time.
It is an advantage of the present invention that better and more timely process control is obtained.
It is yet another advantage of the present invention that there is less interruption of production with reduced down time, resulting in more efficient operation.
It is a feature of the present invention that a plurality of gratings having different pitch and orientation are used.
It is another feature of the present invention that the intensity of the images of each of the plurality of gratings is obtained in at least two planes.
These and other objects, advantages, and features will become readily apparent in view of the following detailed description.